Electron emission devices, such as thermionic emitters, cold cathode field emitters and the like, are currently used as electron sources in x-ray tube applications, flat panel field emission display applications, microwave amplifier applications, electron-beam lithography applications and the like. Typically, thermionic emitters, which operate at relatively high temperatures and allow for relatively slow electronic addressing and switching, are used in x-ray imaging applications. It is desirable to develop a cold cathode field emitter that may be used as an electron source in x-ray imaging applications, such as computed tomography (CT) applications, to improve scan speeds, as well as in other applications. Moreover, applications like low pressure gas discharge lighting and fluorescent lighting, which are limited by the life of the thermionic emitters that are typically used, will benefit from cold cathode field emitters.
Conventional cold cathode field emitters generally include a plurality of substantially conical or pyramid-shaped emitter tips arranged in a grid surrounded by a plurality of grid openings, or gates. The plurality of substantially conical or pyramid-shaped emitter tips are typically made of a metal or a metal carbide, such as molybdenum (Mo), tungsten (W), tantalum (Ta), iridium (Ir), platinum (Pt), molybdenum carbide (Mo2C), hafnium carbide (HfC), zirconium carbide (ZrC), niobium carbide (NbC) or the like, or a semiconductor material, such as silicon (Si), silicon carbide (SiC), gallium nitride (GaN), diamond-like C or the like, and have a radius of curvature on the order of about 20 nm. A common conductor, or cathode electrode, is used and a gate dielectric layer is selectively disposed between the cathode electrode and the gate electrode, forming a plurality of microcavities around the plurality of substantially conical or pyramid-shaped emitter tips. Exemplary cathode electrode materials include doped amorphous Si, crystalline Si and thin-film metals, such as Mo, aluminum (Al), chromium (Cr) and the like. Exemplary gate dielectric layer materials include silicon dioxide (SiO2), silicon nitride (Si3N4) and alumina (Al2O3). Exemplary gate electrode materials include Al, Mo, Pt and doped Si. When a voltage is applied to the gate electrode, electrons tunnel from the plurality of substantially conical or pyramid-shaped emitter tips.
The key performance factors associated with cold cathode field emitters include the emitter tip sharpness, the alignment and spacing of the emitter tips and the gates, the emitter tip-to-gate distance, and the emitter tip density. For example, the emitter tip-to-gate distance partially determines the turn-on voltage of the cold cathode field emitter, i.e., the voltage difference required between the emitter tip and the gate for the cold cathode field emitter to start emitting electrons. Typically, the smaller the emitter tip-to-gate distance, the lower the turn-on voltage of the cold cathode field emitter and the lower the power consumption/dissipation. Likewise, the emitter tip density affects the footprint of the cold cathode field emitter.
Conventional cold cathode field emitters may be fabricated using a number of methods. For example, the Spindt method, well known to those skilled in the art, may be used (see U.S. Pat. Nos. 3,665,241; 3,755,704; and 3,812,559; and C. A. Spindt “A Thin-Film Field-Emission Cathode,” J. Appl. Phys., 1968, vol. 39(7), pp. 3504-3505). Generally, the Spindt method includes masking one or more dielectric layers and performing a plurality of lengthy, labor-intensive etching, oxidation and deposition steps. Residual gas particles in the vacuum surrounding the plurality of substantially conical or pyramid-shaped emitter tips collide with emitted electrons and are ionized. The resulting ions bombard the emitter tips and damage their sharp points, decreasing the emission current of the cold cathode field emitter over time and limiting its operating life. In general, the emitter tip-to-gate distance is determined by the thickness of the dielectric layer disposed between the two. A smaller emitter tip-to-gate distance may be achieved by depositing a thinner dielectric layer. This, however, has the negative consequence of increasing the capacitance between the cathode electrode and the gate electrode, thus increasing the response time of the cold cathode field emitter. One or both of these shortcomings are shared by the other methods for fabricating conventional cold cathode field emitters as well, including the recent chemical-mechanical planarization (CMP) methods (see U.S. Pat. Nos. 5,266,530, 5,229,331 and 5,372,973) and the recent ion milling methods (see U.S. Pat. Nos. 6,391,670 and 6,394,871), all of which produce a plurality of substantially conical or pyramid-shaped emitter tips. Generally, optical lithography and other methods are limited to field openings on the order of about 0.5 microns or larger and emitter tip-to-gate distances on the order of about 1 micron or larger.
Thus, what is still needed is a simple and efficient method for fabricating a cold cathode field emitter that includes a plurality of emitter tips that are continuously sharp. What is also still needed is a method for fabricating a cold cathode field emitter that has a relatively small emitter tip-to-gate distance, providing a relatively high emitter tip density. Such cold cathode field emitters should be suitable for use in x-ray applications, lighting applications, flat panel field emission display applications, microwave amplifier applications, and the like.